This invention relates to sealing apparatus which is particularly suitable for use in a pouch making machine of the type in which a flexible web of heat-sealable material is advanced and folded upwardly to form two face-to-face strips which are interconnected by a bottom fold. More specifically, the invention relates to apparatus for forming a heat seal along the bottom fold or margins of the strips as the strips are advanced along a predetermined path at high speed and with continuous motion as opposed to slower intermittent or step-by-step motion.
The invention has even more specific reference to sealing apparatus comprising a drum mounted to rotate about an upright axis and carrying a heated sealing ring on its lower end. The strips are guided around an arcuate portion of the drum and, as the strips are advanced, the drum is rotated. During advance of the strips and rotation of the drum, the lower margins of the strips are pressed together and one of the strips is pressed against the heated sealing ring. The applied heat and pressure seals the bottom margins of the strips to one another to form a bottom seal along the strips.
Difficulty has been encountered in forming a bottom seal which is free of wrinkles. When the flexible material is heated while being advanced, the material tends to stretch or grow in size to a point where the elastic limit of the material may be exceeded. As a result of the stretching, bubbles and wrinkles are formed in the material as it is heated and, when the material is subsequently cooled, it is left with voids which cause the seal surfaces to be wrinkled.